Is possible to keyframe a mask textures as a factor for two other textures in realtime?
You must be doing something wrong. If you make 2 material and a mask texture, then make a third material that blends the 2 materials using the Mix Color node with the texture as the Fac input the mask should be applied correctly.
Between the UV output of the Geometry node and the Vector input of the Texture node add a Mapping node. That node has support for adding keyframes to any value.
What you want is to add keyframes to the location value and animate the value over multiple frames.
I tested it and it work, here is the file: